(i) Field of the Invention
This invention relates to a rotational-to-linear motion conversion mechanism and, more particularly, to a rotational-to-linear motion conversion mechanism enabling suppression of interference in an engagement area for an improvement in reliability.
(ii) Description of the Related Art
A known one of the rotational-to-linear motion conversion mechanisms of mutually converting the motion direction between the rotational motion and the linear motion includes: a rack rod having a thread formed on the outer peripheral surface; a holder member provided rotatably and movably in the axial direction relative to the rack rod; and a revolving roller that is rotatably supported in the holder member and has a ring-shaped groove formed in the outer peripheral surface to engage with the thread and additionally is placed at an axial angle equal to or greater than the lead angle of the thread with respect to the axis of the rack rod.
In this mechanism the tangent planes of the thread and the ring-shaped groove in the engagement site to each other are perpendicular to the axis of the revolving roller, and the surfaces of them in the engagement site are curved with respect to each other, thus enabling suppression of interference in places except the predetermined engagement site to produce high thrust. Such a mechanism is disclosed in, for example, JP-A No. 2007-120659.
JP-A No. 2007-120659 discloses the curved faces of the thread and the ring-shaped groove in the engagement site, but does not describe the shape of the curved face in detail. Merely forming the curved faces in the engagement site leads to a reduction in conversion efficiency. This is because, for example, when a great load is applied to the mechanism, deformation appears in the engagement site, which increases the area of the contact face (multi-point contact), causing induction of slip. Once slip is produced, the engagement face is roughened, further inducing slip.